The present invention relates to a method of producing a porous material having open pores. More particularly, the invention is concerned with a method of producing a porous material having open pores from a mixture which comprises (i) a glycidyl type epoxy resin, (ii) a first hardener selected from the group consisting of modified polyamine hardeners each being prepared by modifying a polyamine hardener with a modifier, amine hardeners and mixtures thereof, (iii) a second hardener selected from the group consisting of polyamide hardeners, (iv) a filler and (v) water.
Hitherto, for producing a porous material having open pores for use as a filtering medium, air diffusion medium, casting mold, carrier for catalyst and so forth, various methods have been proposed such as sintering of metal powder, sintering of powdered thermoplastic resin, sintering of inorganic powder, hydration setting of cement or the like, pressing or stamping of a mixture of a thermosetting resin and a filler, hardening of a resin liquid containing pore-forming agent followed by the removal of the pore-forming agent by dissolving, extraction or evaporation, use of a foaming agent, and polymerization for hardening of a W/O emulsion of, example, a polyester resin followed by evaporation of water from the hardened mass.
These known methods of producing porous material having open pores, however, encountered one or more of the following problems in connection with the manufacturing process. First of all, it is to be pointed out that these known methods impractically limit or restrict the shape and size of the product. In addition, these methods often require a heat treatment at high temperature, as well as press work at high pressure. The method which makes use of the pore-forming agent require a step of heating or vacuum operation for the removal of the pore-forming agent by evaporation. Furthermore, these known methods have disadvantages in that it is difficult to control the pore size or pore diameter, or that the production steps involved therein are generally complicated and difficult to conduct.
Among the aforementioned problems, the control of the pore size or pore diameter is a particularly important problem for the following reasons. (i) For instance, when the porous material having open pores is used as a material for a filtering medium or casting mold, the principal function thereof is to separate the particles contained in a fluid fIowing therethrough into a first group which passes through the pores of the porous material and a second group which does not pass through the pores of the porous material. This function is determined principally by the diameter of the pores of the porous material. (ii) On the other hand, when the porous material having open pores is used as a material for air diffusion medium, the size of air bubbles and the required pressure for forming air bubbles, which are the two important parameters in operation, are determined by the diameter of the pores of the porous material. (iii) When the porous material having open pores is used as a carrier for a catalyst, the contact surface area of the catalyst is determined by the pore size of the porous material, the contact surface area of the catalyst being the most important factor governing the catalytic action. By the use of the aforementioned known methods, it is quite difficult to control the pore size for the following reasons.
In the production of a porous material from metal powder by sintering, it is difficult to obtain a pore size smaller than 5 microns because of a specific relation existing between the particle size of the metal powder and the surface energy during the sintering step. Consequently, it is quite difficult to effect control of the pore size to obtain pores on the order of 1 micron or so.
Likewise, in the resin powder sintering method or the inorganic powder sintering method, it is difficult to control the packing density at the molding step although the particle size of the used resin powder or inorganic powder may be controlled. In addition, these known methods suffer from a large change in the pore structure at the sintering step, leading to the result that the pore size is distributed over a wide range to hinder the control of the pore size.
As regards the method of mixing a filler with a thermosetting resin followed by pressing or stamping of the mixed material, it is difficult to control the pore size uniformly throughout a product having complicated shape due to uneven molding pressure applied on the different parts of the molded mass or uneven packing of the mixed material, or due to uneven distribution of the thermosetting resin in the mixed material.
In the case of the hydration hardening of a cement or gypsum, a difficulty is involved in the control of the nucleation and growth of the crystals of the hydrate, and precise control of the mean pore size to obtain pores having an average pore diameter on the order of 0.2 to 10 microns cannot easily be effected.
With regard to the method in which the pore-forming agent is evaporated and removed from the hardened resin containing the pore-forming agent, a technique has been proposed for the production of a thin film having pores having an average pore diameter ranging from 0.01 to 0.1 micron. This method, however, cannot be applied to the production of a product having considerably large size and thickness. In the method in which a resin in the form of an emulsion type is hardened followed by the evaporation or extraction of the dispersoid, it is not easy to control the size of the dispersoid, and many disconnected or closed pores are inevitably formed. The control of the pore sizes on the order of 0.2 to 10 microns as a mean is also difficult in this method.
In order to overcome the aforementioned problems and to produce a large porous product having open pores and relatively complicated shape, an improved method has been proposed by Japanese Patent Publication No. 2464/1978 whereby a porous material having precise dimensions and open pores of desired diameter can be produced. This improved method known from the prior patent publication comprises the steps of preparing an O/W emulsion slurry from a mixture containing a glycidyl type epoxy resin, a polymeric fatty acid polyamide hardener, a filler and water, casting the slurry in a water-impermeable mold, hardening the slurry while it contains water, and dehydrating the hardened mass, whereby the desired object is attained.
According to the method known by Japanese Patent Publication No. 2464/1978, the pore size of the porous material having open pores may be controlled by varying the particle size of the filler, by varying the added amount of the reactive diluent, or by varying the mixing ratio of the epoxy resin, hardeners, filler and water. However, the pore size cannot be controlled within a wide range, since the variable range of the amount of reactive diluent and the variable range of the mixing ratio of the epoxy resin, hardeners, filler and water are limited in view of the contraction or shrinkage at the hardening step and the need to satisfy the required strength of the hardened product. It is thus necessary that the particle size of the used filler be varied in order to control the pore size within the range of 0.5 to 10 microns. However, the strength of the hardened product is disadvantageously lowered when a filler having coarser particle size is used. On the other hand, if the content of water in the slurry is decreased to prevent reduction in strength, the viscosity of the slurry becomes high. A further disadvantage of the use of a coarser filler is that the filler particles suspended in the slurry tend to precipitate to lower the operability.
In order to solve the aforementioned problems and to provide a method of producing a porous product having open pores and having a large and complicated shape with good accuracy in dimension, U.S. Pat. No. 4,464,485 discloses a method of producing a porous material having open pores having an average pore diameter of more than 1.5 microns. In this known method, the desired object is attained either (i) by the use of a hardener which is obtained by changing the mixing ratio of an amide compound obtained through a reaction between a monomeric fatty acid and an ethyleneamine represented by the formula of H.sub.2 N--(CH.sub.2 --CH.sub.2 --NH).sub.n --H where "n" is 3 to 5, and a polymeric fatty acid polyamide obtained through a reaction of a polymeric fatty acid and the aforementioned ethyleneamine, or (ii) by the use of a hardener which is obtained by changing the mixing ratio of the monomeric fatty acid and the polymeric fatty acid followed by reacting the fatty acid mixture with the aforementioned ethyleneamine. (This method will be hereinafter referred to as "the method of changing the mixing ratio between monomeric fatty acid and polymeric fatty acid".)
However, "the method of changing the mixing ratio between monomeric fatty acid and polymeric fatty acid" to control the pore size has the following two disadvantages.
(i) In order to produce a hardened product having large size pores by this known method, the ratio of the monomeric fatty acid must be increased. However, a porous product having open pores prepared from such a composition suffers considerable increase in contraction or shrinkage at the hardening step. The upper limit of the pore size is about 5 microns if contraction is to be suppressed within a tolerable range. It is thus difficult to produce a porous product having open pores of larger size and having precisely controlled dimensions. This is the first disadvantage of "the method of changing the mixing ratio beween monomeric fatty acid and polymeric fatty acid".
(ii) In order to produce a hardened product having small size pores by this known method, the ratio of the polymeric fatty acid must be increased. However, the change in pore size of the hardened product is little within the mixing ratio range where the ratio of polymeric fatty acid in the fatty acid mixture is large. Accordingly, the pore size cannot be varied in compliance with the requirement only by changing the mixing ratio between the monomeric fatty acid and the polymeric fatty acid. Subsequently, when it is desired to produce a hardened product having a relatively small pore size, a filler having a small particle size must be used. For the production of hardened products having pore sizes varied within a small pore size range, a number of fillers having different particle sizes must be prepared. However, much time and labor are required for the preparation of fillers having different particle sizes including pulverization of the filler and separation or grading of the pulverized filler particles. When it is intended to vary the pore size of the hardened product by using a single kind of filler and by varying the kind of hardener, the minimum pore size is about 1.5 microns when the single filler is selected so that the maximum pore size obtainable by the use thereof is, for example, about 5 microns. A porous product having pores of smaller pore size cannot be obtained by the use of such a single filler. This is the second disadvantage of "the method of changing the mixing ratio between monomeric fatty acid and polymeric fatty acid".